This invention relates to novel membranes of substantially improved transfer properties. The invention relates particularly to improved ion exchange membranes and methods of their preparation and also to improved apparatus utilizing stacked cells of membranes, e.g. fuel cells, water electrolysis apparatus, and such electrochemical manufacturing cells as chlor alkali cells.
Solid-polymeric ion exchange membranes are well known in the art. In general, they are formed of a backbone polymer system which comprises an inert chain and branches with functional groups capable of exchanging anions with cations or vice versa. These membranes are utilized in a wide variety of applications including, fuel cells, electrochemical production cells such as caustic chlorine cells, water electrolysis cells, high-energy density batteries and the like. Non-ionic membranes are also used in various reverse osmosis or ultrafiltration processes, e.g. water desalting, electropaint recovery processes, cheese whey processing, water pollution control, by-product recovery and numerous other applications requiring removal of some kinds of molecules from solutions. Among the better known ion exchange membranes are those represented by the perfluorosulfonic acid membranes available from E. I. duPont DeNemours & Co. (Dupont) under the trademark NAFION. These membranes are well known in the art. See "Nafion", an Electrochemical Traffic Controller, pages 10-13 of DuPont Innovation Volume 4 No. 3 (1973) by Vaughan.
This material is reported to be a copolymer of tetrafluoroethylene (TFE) and a sulfonyl fluoride vinyl ether (PSEPVE).
Other well known ion exchange membranes which may be processed according to the invention are based on high density polyethylene skeletons and those based on polyvinylidene fluoride or polyvinylfluoride skeletons. Asahi Chemical Industry supplies ion exchange membranes having a styrene-divinyl benzene (DVB) base and another based on perfluorocarboxylic acid polymeric structure.
A problem that has been virtually universal in the development of ion exchange membranes has been the necessity of balancing a desirably low fluid transfer (mass transfer) characteristic against other characteristics of the membrane. In ion exchange membranes, the balance has typically been made with attention to the maintenance of good electroconductivity for the membrane, and this requirement has tended to limit the extent to which low electroresistivity characteristics can be achieved.
As will be described below, the present inventor has directed his attention to the provision of ion exchange membranes which have both improved low permeability and low electrical resistivity characteristics. In so doing, he has also provided means to provide improved reverse osmosis, ultrafiltration, and dialysis membranes.